A nurse has been charged with the murders of eight elderly people at nursing homes in Canada over a seven-year period.

Woodstock Police chief William Renton said Elizabeth Tracey Mae Wettlaufer, 49, was charged with first-degree murder in the killings that took place in south-western Ontario between 2007 and 2014.

"The victims were administered a drug. We're not in a position at this time to comment further on the specifics of the drug as it forms part of the evidence that is now before the courts," Ontario Provincial Police detective Dave Truax said.

Mr Truax would only say that a number of drugs were stored and accessible in the nursing homes where the suspect worked.

Police said Wettlaufer appeared in court on Tuesday morning and remained in custody. The investigation is ongoing and officials said more charges could be brought in the future.

Woodstock Police said they did not know whether Wettlaufer was represented yet by a lawyer.

Ontario Premier Kathleen Wynne called the alleged murders by a nurse "extremely distressing and tragic".

Wettlaufer, of Woodstock, was employed by Caressant Care Nursing and Retirement Homes, which operates 15 facilities in small Ontario towns.

Police said seven of the victims died at a Caressant nursing home in Woodstock, a community of 37,000 people about halfway between London and Hamilton, Ontario.

She was also employed at the Meadow Park facility in London, where the other victim died.